The present invention generally relates to a medical device capable of detecting the presence of electromagnetic interference (EMI) and effectively cancelling the interference to prevent it from adversely affecting the detection of other electrical signals.
During operation, various medical devices transmit and receive electrical signals which can be interfered with due to the presence of EMI. An example of one such medical device is a Doppler flowmeter used to detect the magnitude of blood flow during surgery, for example. Such a flowmeter, which is disclosed in U.S. Pat. No. 5,368,554 to Nazarian, et al., operates by periodically transmitting an ultrasonic signal through flowing blood via a transmitter, receiving an ultrasonic signal reflected from the transmitted signal via a receiver, and converting the received signal into an electrical signal of the same frequency. The frequency of the received signal differs from the frequency of the transmitted signal by an amount which corresponds to the magnitude of the blood flow, due to the well-known Doppler shift phenomenon.
The operation of such a flowmeter can be disrupted by the presence of EMI, causing the flowmeter to generate erroneous blood flow readings. One example of EMI is "Bovie" interference, which is caused by the use of an electro-surgical unit during surgery. In such a unit, high-energy radio-frequency ("RF") radiation is used to heat a metal tip of the unit used for cauterizing purposes. The RF radiation produced by such a unit can severely disrupt the operation of a Doppler flowmeter for a number of reasons. First, the intensity of the RF radiation generated by an electro-surgical unit is typically much greater than that of the electrical signals from which the blood flow readings are determined. Second, the receiver is typically connected to the Doppler flowmeter via a relatively long cable, which tends to act as an antenna, thus attracting the RF radiation and aggravating the interference with the relatively weak signal the Doppler flowmeter is trying to detect. Finally, the frequency of the RF radiation emitted by an electro-surgical unit may vary over a relatively wide frequency band, and the frequency band typically varies with the particular electro-surgical unit used, which makes attempts to reduce the effects of such radiation by using conventional electromagnetic-shielding techniques and filtering difficult, if not impossible.